1. Field of the Invention
The present invention relates to a semiconductor acceleration sensor device having an acceleration sensor chip mounted on a package.
2. Description of the Related Art
A typical conventional semiconductor acceleration device has an acceleration sensor chip for outputting an electric signal in accordance with a magnitude and direction of applied acceleration, and the acceleration sensor chip is mounted on a package. Various types of semiconductor acceleration sensor devices have been known in the art. One example is a semiconductor acceleration sensor device having the structure shown in FIG. 17 of the accompanying drawings. In this semiconductor acceleration sensor device, the acceleration sensor chip 4′ shown in FIG. 16 of the accompanying drawings is mounted on the package 1′. This conventional acceleration sensor device will be described with reference to FIG. 16 and FIG. 17.
The acceleration sensor chip 4′ includes a frame that has four beams 18′ extending inward. A weight 16′ is supported at the center of the frame by the four beams 18′. A piezoresistance element 20′ is attached to each beam 18′. Because of this structure, if the weight 16′ moves in the X, Y or Z direction, some beams 18′ stretch and other beams 18′ shrink, and the resistance values of the piezoresistance elements 20′ change. The piezoresistance elements 20′ in the respective axis directions constitute a bridge circuit, and the changes of the resistance values are output as electric signals and computed. Accordingly, the direction and magnitude of the applied acceleration can be detected. This type of acceleration sensor is disclosed in, for example, Japanese Patent Application Kokai (Laid-Open) No. 2004-198243.
Because the acceleration sensor chip 4′ has a mechanism to sense the movement of the weight 16′, the package 1′ for mounting the acceleration sensor chip 4′ must be hollow inside.
The acceleration sensor chip 4′ is adhered to the bottom face 5′ inside the package 1 by a low elasticity element 3′, such as silicon rubber, which has adhesiveness. The low elasticity element 3′ is used because the shock resistance of the acceleration sensor chip 4′ is secured. The low elasticity element 3′ is formed by coating a certain liquid on the bottom face 5′ and curing it.
Another example of the semiconductor acceleration sensor device is disclosed in Japanese Patent No. 3517428.
In the above described conventional semiconductor acceleration sensor devices, the thermal expansion coefficient of the low elasticity element for adhering the acceleration sensor chip to the bottom face inside the package is greater than the thermal expansion coefficient of the acceleration sensor chip. As a result, distortion is generated in the acceleration sensor chip if the ambient temperature changes. If distortion is generated in the acceleration sensor chip, the piezoelements are affected. This affects the detection of acceleration. In other words, acceleration cannot be detected accurately if the temperature changes.